1. Field of the invention
The present invention relates to a memory, and more particularly to a non-volatile memory having three data states and a method for manufacturing the same.
2. Description of the Prior Art
As generally known in the art, in the case of conventional volatile memories including DRAMs and SRAMs and non-volatile memories including flashes, each cell has one of two value states of “0” and “1.” Specifically, in the case of DRAMs, each cell has one of two data states of “0” and “1” depending on whether it has stored electrical charge in the capacitor or not. In the case of SRAMS, each cell has one of two data states of “0” and “1” depending on whether it is latched or not. In the case of flash memories, as shown in FIG. 1, each cell has one of two data states of “0” and “1” depending on whether it has injected electrons into the floating gate 3a or not, which corresponds to each of two threshold voltages.
It can be understood, therefore, that the capacity of conventional volatile and non-volatile memories is equal to the number of entire memory cells.
In FIG. 1, reference numeral 1 refers to a silicon substrate; 2 is a device separation film; 3 is a tunnel oxide film made of a silicon oxide film; 4 is a floating gate made of a polysilicon film; 5 is an oxide film for a control gate; 6 is a control gate; 6a and 6b are a polysilicon film and a tungsten silicide film for the control gate, respectively; 7 is a hard mask film; 8 is a spacer; 8a and 8b are an oxide film and a nitride film for the spacer; and 9 is a source/drain region.
It is a widely known fact that, as technologies develop and a larger amount of information needs to be processed, a memory having larger capacity is required. However, there exist many technical limitations in highly integrating memories. Therefore, new structure and process are strongly required to realize a memory having a satisfactorily large capacity.